Get active instruments

Get all active instruments for a specific market with update timestamp.

Response includes:

• updated_at: Unix timestamp in milliseconds of when the instrument list was last updated
• instruments: Array of active instrument names (both options and perpetuals)

Perpetual instruments are always active and available:

• BTC_USDC-PERPETUAL
• ETH_USDC-PERPETUAL
• ARB_USDC-PERPETUAL

Options instruments are returned based on their expiration status and market availability.